Enolate ions, preparation acylation

Furans can be prepared by acid catalyzed cyclization of masked 1,4-diketones. /3-Chloroallyl ketones which are obtained by alkylation of enamines or enolate ions behave as masked 1,4-diketones and afford furans on treatment with acid (67JA4557). 2,4-Dialkyl-furans (40) have been prepared by cyclization of the 3-chloroallyl ketone (39), which may be obtained by acylation of allyl chlorides (73KGS1434). 

In addition to preparation of arylhydrazones from the carbonyl compounds and an arylhydrazine, the Japp-Klingemann reaction of arenediazonium ions with enolates and enamines is an important method for preparation of arylhydrazones. This method provides a route to monoarylhydrazones of a-dicarbonyl compounds from /3-keto acids and to the hydrazones of pyruvate esters from /3 -keto esters. Enamines also give rise to monoarylhydrazones of a -diketones. Indolization of these arylhydrazones provides the expected 2-acyl-or 2-alkoxycarbonyl-indoles (equations 95-97). 

Other derivatives of carboxylic acids and some unusual catalyst systems have found favor. Diarylboryl hexachloroantimonates activate acyl chlorides, carboxylic anhydrides and acyl enolates. A number of metal oxides have been successfully employed. It is worth noting at this point that the chloroacetylium and bromoacetylium ions, which can be prepared in either Freon 113 or sulfur dioxide, are more stable than the acetylium ion and have been shown to give high yields of ketones at low temperatures. ... 

Purpose. This experiment investigates the conditions under which the enol acetate of an a,p-unsaturated ketone is prepared. The combined use of acetic anhydride with chlorotrimethylsilane is an illustration of a technique for the generation of acylium ions, a reactive electrophile for acylation reactions. 

---